The present invention relates to an optical modulation apparatus and an optical modulation method, and more particularly to an optical modulation apparatus and an optical modulation method for controlling modulation drive signals.
Today, research on optical fiber communication systems is actively undertaken to increase the transmission capacity and the transmission distance. As means to raise the transmission speed and the elongation of the transmission distance in particular, waveguide type optical modulators are used. Among waveguide type optical modulators, ones using LiNbO3 for their substrates are well known, and they can be classified into three types: light intensity modulators, optical phase modulators and optical polarization modulators.
Next will be described an optical modulation circuit in which, out of these waveguide type optical modulators, an optical polarization modulator is used. In the optical polarization modulator, there are installed an optical input port to which optical signals are inputted and an optical output port from which polarization-modulated signals are outputted. The optical polarization modulator is also provided with an amplifying circuit for outputting modulation drive signals corresponding to polarization-modulated signals. These modulation drive signals are inputted to an electric signal input port of the optical polarization modulator and, traveling over a strip electrode line inside, outputted from an electric signal output. The modulation drive signals are terminated by a terminating circuit matched to the characteristic impedance of the strip electrode line.
However, optical modulation apparatuses according to the prior involve the problem that they are not disposed to cause the gain of said amplifying circuit to be controlled in any particular way. Therefore, if modulated input signals to the amplifying circuit vary in amplitude or its ambient temperature changes, the amplitude of modulation drive signals supplied from the amplifying circuit will vary. More specifically, a temperature rise would bring down the gain of the amplifying circuit. Where an optical polarization modulator is used as a waveguide type optical modulator, any variation in the amplitude of modulation drive signals would invite a change in the quantity of polarization modulation and a deterioration in the degree of polarization (referred to xe2x80x9cDOPxe2x80x9d, hereinafter). Similarly, a light intensity type waveguide type optical modulator would deteriorate in quenching ratio.
An object of the present invention, attempted to solve the aforementioned problems, is to provide an optical modulation apparatus and an optical modulation method capable of achieving optical modulation characteristics against changes in the amplitude of modulated input signals or in ambient temperature.
An optical modulation apparatus according to the invention is provided with an optical modulator for modulating optical signals, a variable amplifying circuit for amplifying modulated input signals and supplying modulation drive signals to the optical modulator, and a detecting circuit for detecting the modulation drive signals supplied from the optical modulator. It may further be provided with a control circuit for controlling the variable amplifying circuit according to the modulation drive signals supplied by the detecting circuit. An optical modulation method according to the invention is a method to detect modulation drive signals supplied from an optical modulator and controlling an amplifying circuit for supplying the optical modulator with the modulation control signals on the basis of these signals.
The above-mentioned optical modulation apparatus and optical modulation method according to the invention manifest stable optical modulation characteristics.